1. Field of the Invention
The present invention relates to a grinder such as, for example, an internal grinding machine, an cylindrical grinder or a centerless grinder, and a grinding machine such as, for example, a superfinishing machine and, more particularly, to a control device in an automatic shoe-supported milling machine, an improvement in a work exchanger and stabilization of the processing accuracy.
The present invention also relates to a grinding method and a grinding machine, in which the change in processing efficiency of a grindstone (a so-called "grindstone sharpness") resulting from wear of the grindstone is determined during the process and the subsequent milling is controlled according to the determined grindstone sharpness and, more particularly, to the grinding method and the grinding machine which are effectively utilized where the cycle time of the grinding process is short.
The present invention furthermore relates to a grinding method and a grinding machine, in which in the grinding machine of a type having a processing system of a low rigidity, such as an internal grinder, or the grinding machine of a type having a work of a low rigidity and a support system of a low rigidity, the depth of cut is set back to release deflection subsequent to a rough processing in the event of the deflection increasing as a result of a processing force. In particular, the present invention relates to a grinding method and a grinding machine which are effectively utilized where a number of identical works are successively machined.
The present invention relates to a grinding method and a grinding machine, wherein even in the presence of a change in machining allowance and also in grindstone sharpness (processing efficiency) during the final grinding process, the length of time during which the machining is carried out, that is, the machining time, can be controlled to a target value and the processing resistance is controlled to stabilize the processing accuracy.
2. Description of the Prior Art
As is well known to those skilled in the art, machine operations of grinding machines are sequentially controlled by the use of a sequencer. In such case, one machine operation is followed by the subsequent machine operation after completion of such one machine operation has been confirmed by means of an approach sensor or the like. The sequence of control of operation of the internal grinder is shown in FIG. 7 and a time chart thereof is shown in FIG. 8.
The prior art machine operation will be discussed with reference to FIGS. 7 and 8. After completion of the processing, the grindstone is retracted from a cutting position to a retracted position and a grindstone retraction sensor disposed at a position adjacent the retracted position is switched on. An electric signal issued by the grindstone retraction sensor is transmitted to a sequencer to cause the latter to issue a retraction command to retract a grindstone table. When the grindstone table subsequently depresses a table retraction affirm sensor disposed at a position adjacent a retracted position for the grindstone table, the sequencer affirms retraction of the grindstone table and then generates a gauge retraction signal. In the case of the internal grinder, when both of the grindstone and the gauge move out of the work, the loader is brought into operation to discharge the processed work so that a next succeeding work to be processed can be loaded.
Although the loader operates at the discharge position to replace the machined work with a next succeeding work to be machined, the actual loading of the next succeeding work to be machined takes place after arrival at the discharge position has been ascertained and the timer has subsequently timed up. Upon loading of the next succeeding work to be machined, the loading operation is ascertained by means of one or more sensors and the operation then takes place in the order of the gauge, the grindstone table and cutting which is reverse to the unloading. FIGS. 6A and 6B illustrate an exemplary loading apparatus. Actuators or accomplishing those sequential operations are generally employed in the form of hydraulic and/or pneumatic cylinders. Machine arrangement of the inner grinder is such as shown in FIG. 5.
Referring to FIG. 6A, the work W which has been processed is discharged by an unloading operation of a loader arm cylinder 51 and the work W within a pocket 53 defined in a loader arm 52 engaged therewith is moved to a placement position P. The work W which has already been processed is pushed by a next succeeding work W to be subsequently processed towards a stop device 54 at which the next succeeding work W is held still in contact therewith. At this time, in order to ascertain that the processed work W has been replaced with the next succeeding work W, the loader arm 52 is held inoperative by a timer subsequent to a confirmation by a sensor sd and until the next succeeding loading is initiated.
While there is available a machine in which the sequence from the cutting of the work to the placement thereof is carried out by separate mechanisms, the illustrated machine requires the use of separate sensors for actuating a placement mechanism and for ascertaining completion of the placement operation subsequent to confirmation done by the sensor sd.
Referring now to FIG. 6B, when the timer times up, the loader arm cylinder 51 starts its loading operation. The processed work W is removed out of the machine as a result that the stop device 54 synchronized with the operation of the loader arm 52 moved to a retracted position. On the other hand, the work W within the pocket 53 in the loader arm 52 is placed within a work rotating support device 55. This confirmation of the work W having been loaded is accomplished by a sensor se, and respective operation of the gauge 56 and the grindstone 57 are initiated and, after they are moved to a work processing position X0, a further processing is initiated.
After completion of the processing, the loader arm cylinder 51 starts its retraction after, as an indication of both of the grindstone 57 and the gauge 56 having been separated from the work W, an indication of the grindstone 58 having been retracted and an indication of the gauge 56 having been retracted have been entered.
In the above discussed case, the following first problem has been found.
In the above described operation, where the actuator is employed in the form of a hydraulic or pneumatic cylinder, switching of a valve and transmission of a pressure within the piping system take a substantial length of time and a delay necessarily occurs even through a sequence control signal is changed. Because of such a delay, not only does the sequence control require confirmation of the operation, but also difficulty is involved in installing and setting up limit switches required to accomplish confirmation of the operation.
By way of example, the delay of the sequencer runs from a few msec. to some tens msec., the switching of the valve may take some tens msec., and transmission of the pressure in the piping system accompanies not only a delay of about 100 msec., but also a variation in transmission time. In the case of the grinding machine such as shown in FIG. 7, about ten different operations take place and, therefore, even though a delay of individual controllers may be minimal, accumulation of those delays brought about by those controllers would be detrimental. In particular, a timer has been required so that during delivery of the work, the work can assuredly be stabilized at the placement position.
Elements which will bring about a delay may include as follows:
(1) Approach switch used to ascertain operation:
This switch generally has a delay of 1 msec. or less and has a high speed responding capability.
(2) Sequencer I/O signal:
In the case of an AC relay or the like, a delay may result from the power source frequency. The delay of 1/60 sec. at maximum may result in at 60 Hz. Even in the case of a DC relay, a delay of some tens of msec. may result.
(3) Sequencer processing time:
Although variable subject to the sequencer and the method of compiling its program, a delay of a few msec. or larger may result.
(4) Response of an electromagnetic valve:
Movement of a spool in the electromagnetic valve requires about some tens of msec.
(5) Transmission of a pressurized fluid:
Although variable subject to the length of the piping system, the rigidity of pipes and the difference in hydraulic or pneumatic pressure, a delay of some tens of msec. or larger may result.
As discussed above, the conventional loading apparatus and other machine operations necessarily involve a delay and a variation. Though attempts have hitherto been made to employ a servo valve for the hydraulic valve, the delay in the basic sequential control could not be removed. The details of the idle time (non-grinding time) of the grinding machine as far as the items (2) to (5) above are concerned are such as shown in FIG. 9.
Although the individual delay elements are small, a delay of about 1 sec. would occur when the operation is repeated seven times. Accordingly, it is one of the major factors that require improvement in the processing machine.
In order to minimize the delays, it may be contemplated to employ a servo system for the actuator which tends to bring about the smallest delay, and a grinding machine is currently available of a type in which the delay has been reduced by the use of a hydraulic servo system. However, what is provided with a servo system is only the loader arm which requires force, and even the hydraulic servo system requires a delay of some tens of msec. or large before it starts its operation.
The prior art associated with the grindstone sharpness will now be discussed.
The grindstone sharpness (the processing efficiency) tends to vary as it wears during the grinding cycle, and the value of the grindstone sharpness is one of the important factors to accomplish a control of cutting.
The grindstone sharpness is evaluated with .LAMBDA., which is expressed by the following formula, and the reciprocal of .LAMBDA.. EQU .LAMBDA.=(Processing Force)/(Processing Efficiency Z)
In other words, the ratio of the processing force relative to the processing efficiency (Amount of works removed per unitary time) represents the grindstone sharpness. The processing force referred to above is represented by a value such as the orthogonal grinding force Fn(N), the tangential grinding force Ft(N), the grinding power P(kW) or the like. The unit of the processing efficiency Z is mm.sup.3 /sec., or mm.sup.3 /mm.sec.
In the above formula, If the parameter .LAMBDA. is large, the processing efficiency for a given processing force is low and, hence, the sharpness of the grindstone is low. On the other hand, if the parameter .LAMBDA. is small, a relatively large amount of material can be removed with a low processing resistance and, hence, the sharpness of the grindstone is considered favorable.
To evaluate the sharpness, both of the processing force and the processing efficiency must be detected, and the processing force can be determined in reference to a sensor signal indicative of the cutting deflection or the grinding power. The processing efficiency can, on the other hand, be determined by the utilization of a signal of an in-process gauge effective to detect the dimension of the work being processed.
With respect to the evaluation of the sharpness, reference will now be made to FIGS. 15 and 16. FIG. 15 illustrates an arrangement of equipments of the internal grinding machine being operated and FIG. 16 illustrates a condition of the machining process.
As shown in FIG. 15, the work W to be processed is mounted on shoes 6a and 6b and a driving plate 116 for rotation together therewith. The grindstone 4a is positioned inside the work W to be processed and performs a cutting in a direction transverse to the work W while being rotated. The dimensions to which the work W is to be processed are captured by a gauge contact (a detector support arm) 10a within the work W and are measured by an in-process gauge 10. The processing force (the grinding force) is measured by a sensor 119 for detecting deflection of a grindstone drive motor axis (not shown) or a deflection of a grindstone axis. At this time, the processing position (the processing point) of the grindstone 4a and the point of measurement by the in-process gauge 10 do not match with each other and, therefore, a possible error would occur in the measurement of the in-process gauge 10 as a result of a thermal expansion of the work W.
The processing process will be discussed. Referring to FIG. 16, when the rough processing is initiated during which cutting X (=advancing motion of the grindstone) takes place, the processing force P increases accompanied by a change in processing dimensions (measured values) g. Although the processing force P attains a predetermined value at the time of completion of the rough processing, a frictional heat resulting from grinding penetrates the work and, therefore, the processing dimensions would be greater than expected. While the processing dimensions that can be measured is expressed by g, the actual processing dimension are expressed by g-.sigma. (shown by the dotted line) because the dimensions containing a thermal expansion .sigma. taking place in the work (shown in FIG. 16 with the axis of ordinates expanded) are measured.
The quantity of the thermal expansion .sigma. of the work accompanies a considerable delay in time as compared with the change of the processing force and considerable expansion and contraction take place during the processing as shown therein. By way of example, the thermal expansion and the thermal contraction take place more than 10 .mu.m in the case of an oil-based coolant or 5 .mu.m in the case of a water-based coolant.
The above discussed case involves the following second problem:
Although the processing force can be obtained by measuring the grinding force and the grinding resistance, the processing efficiency can be obtained by the following equation in which D represents the diameter of the work to be processed and any influence brought about by the thermal expansion of the work is neglected. EQU Z.apprxeq..pi..times.D.times.(dg/dt) mm.sup.3 /mm.sec)
For this reason, no accurate evaluation of the grindstone sharpness is possible. In particular, since the thermal expansion and contraction take place considerably in the work W from a time before completion of the rough processing and also during the finishing process and, therefore, the extent of inaccuracy of the measured value of the grindstone sharpness (the processing efficiency) is considerable.
Although any error in grindstone sharpness will not pose any greater problem in the case of a low-speed processing, an accurate measurement of the grindstone sharpness is necessary where a large number of works such as, for example, bearing races are processed at a high speed and, at the same time, must satisfy severe requirements for the accuracy. In particular, where the cutting control to be employed during the finishing process or the like is to be tailored, no stable control is possible unless the accurate grindstone sharpness is obtained.
Hereinafter, the prior art related to the bite retraction will be discussed.
To process the single work with the grinding machine, the rough processing and the finishing process are carried out successively to secure the processing efficiency and the processing accuracy. Where, for example, the internal grinding machine having a grinding system of a relatively low rigidity is employed, the bite retraction in a small quantity is carried out subsequent to the rough processing and the finishing process follows by releasing a deflection in the grinding system. In flus way, by effecting the bite retraction prior to the finishing process, the time required to finish the work can be shortened.
A condition of a deflection occurring in the grinding system is exaggeratedly shown in FIG. 17. In the case of the internal grinding, the grindstone axis 109a deflects under the influence of the processing force and, with cutting X1(t) an uncutting of a magnitude corresponding to the deflection .delta. will result in the work W. The processing dimensions X2(t) is a function of the depth of cut X1(t) and the grinding time constant X and is expressed as follows: EQU dX2(t)/dt=(1/.tau.).multidot.(x1(t)-X2(t) (1)
The grinding time constant .tau. referred to above varies depending on the grindstone sharpness (the processing efficiency), the material of the work to be processed, the shape of the work and so on.
The processing conditions (processes) in which the deflection is released by accomplishing the bite retraction and in which it is not released, respectively will be discussed by referring to the comparison between FIGS. 18A and 18B.
In order to secure the grinding accuracy, it is necessary to maintain the deflection .delta.(t) at the termination of the cutting at a predetermined value or less. Where no bite retraction is effected, the length of time corresponding to three times the grinding time constant is needed to restore the deflection that takes place during the finishing process. On the other hand, where the bite retraction is effected, it is possible to effect the cutting more than expected during the rough processing and, since the deflection can be restored before the finishing process, the deflection can be quickly restored during the finishing process. Thus, the process time can be shortened.
With the grinder hitherto available, two exemplary methods of determining the amount of the bite retraction are employed: One is to determine the amount of the bite retraction by repeating grinding experiments so that the processing cycle and the processing accuracy can be stabilized and this method is largely employed. The other is to accomplish an automatic bite retraction wherein, if the processing force and the processing power are controlled, for example, in the case of the power control, the following equation is assumed;
Amount of Bite retraction Xbo=Control System Constant.times.(Pr-Pf) wherein Pr represents the power (kW) set during the rough processing and Pf represents the power (kW) set during the finishing process.
However, with these amounts of the bite retraction, the cycle tends to become unstable if the speed of cutting during the finishing process is decreased and/or the power set during the finishing process is lowered, accompanied by considerable variation in time required to accomplish the finishing process. For this reason, the finishing allowance and the cutting time must be increased so that the amount of the bite retraction can be decreased. Also, even in the case where the sharpness of the grindstone such as a CBN grindstone tends to vary considerably before and after dressing and/or the processing allowance tends to vary, the cycle tends to become unstable.
The above discussed case involves the following third problem:
In view of the foregoing situations, it is desired to develop a method of determining the amount of the bite retraction with which even in the presence of the above discussed reasons for the instability, a stabilized grinding cycle can be accomplished. Therefore, the following method of determining the bite retraction has been conceived.
The basic characteristic of the grinding process can be expressed as follows:
Speed of Growth of Work Dimensions: V(t)=dX2(t)/dt, and
Grinding Deflection: .delta.(t)=X1(t)-X2(t).
Therefore, the equation (1) referred to hereinbefore can be rewritten as follows. EQU V(t)=.delta.(t)/.tau. (2)
This can be construed as .delta.(t)=.tau..multidot.V(t) and, therefore, the grinding deflection is equal to the product of the speed of growth of the dimensions of the work (which may be substantially equal to the cutting speed, dX1(t)/dt, if the deflection is stabilized) times the grinding time constant.
The amount of the bite retraction is used to render the rough grinding deflection to be the finishing grinding deflection. Accordingly, if during the rough processing the speed V(t) of growth of the work dimension used in the equation (2) above or both of the cutting speed dX1(t)/d and the grinding time constant .tau.(t) are available, the deflection .delta.(t) can be calculated and the optimum amount Xbo of the bite retraction at which the process goes onto the finishing process can also be calculated.
However, in performing the control by the utilization of the amount of the bite retraction Xbo so calculated, there is a third problem in that no NC device is available which has a capability of changing the amount of the bite retraction during the cutting. The NC device has a capability that in order to determine the path at the time of start of the processing, the speed can be changed by an override, but nothing is available which can change the position during the processing.
For this reason, it is necessary to develop a NC device of a type in which the amount of the bite retraction after completion of the rough processing can be changed during the rough processing.
Also, since the finishing process is predicated to achieve the control during the rough processing, a delay in the control system and also in the mechanical system poses a considerable problem. Nevertheless, since an abrupt change of the grinding time constant does not occur so often, the value of the previously processed work during the processing of such work can be used, but it is desirable to determine the grinding time constant of the work being currently processed in order to accomplish an improvement in accuracy.
In view of the above, an in-process measuring method of obtaining the grinding time constant of the work being currently processed will be considered. During the grinding, the grindstone sharpness tends to vary as discussed in connection with the second problem. Change in grindstone sharpness result in change of the grinding time constant and in turn change in control gain of the grinding system. In the case where the processing process is to be controlled, it is necessary to accurately grasp this change.
The grinding time constant .tau. is expressed as follows:
.tau.=.alpha./[(Rigidity in Grinding System Kg).times.(Grindstone Sharpness .LAMBDA.)]
wherein .alpha. represents the constant determined by the work.
.LAMBDA.=(Grinding Force Fn(N))/(Processing Efficiency Z (mm.sup.3 /sec))
In other words, the grinding time constant .tau. is inversely proportional to the grindstone sharpness A.
Where the same works are continuously processed, the constants .alpha. and Kg may be considered to be the respective constant values and, once the grindstone sharpness .LAMBDA. is available, the grinding time constant .tau. can be fixed.
Change in grindstone will now be considered. Assuming that the grinding time constant is .tau.0 at a reference grindstone sharpness A0, the grinding time constant .tau.t when the grindstone sharpness attains Aa during the processing can be expressed as follows: EQU .tau.t=.tau.0.times.(Aa/A0)
The second problem is associated with the manner by which the grindstone sharpness during the processing is determined.
A method of calculating the cutting speed V(t) will now be described. The cutting speed V(t) is readily understood as meaning a speed of processing the workpiece which is expressed by dX2(t)/dt. Where an in-process gauge is employed, the cutting speed can readily be obtained by differentiating the dimension signal.
Where no in-process gauge is employed and only the power or the processing force is detected, it can be obtained by the following manner. Namely, the deflection .delta.(t) in FIGS. 18A and 18B is the same as the grinding power and the grinding resistance and, since when it becomes an ordinary condition, dX1(t).apprxeq.dX2(t), dX1(t)/dt can be obtained by determining that the power or the processing force attains an ordinary condition.
The method of determining the amount of the bite retraction will now be considered. If the grinding time constant .tau. and the cutting speed V could be detected, the grinding deflection .delta.(t) can be calculated by the equation, .delta.(t)=.tau..times.V(t). It is recommended to use this grinding deflection .delta.(t) as the amount of the bite retraction.
If the NC device of a type in which the amount of the bite retraction after completion of the rough processing can be changed during the rough processing as discussed in connection with the third problem discussed above could be developed, during the rough processing a preset value of the NC device is re-memorized so that the value of the previously discussed grinding deflection .delta.(t) may represent the amount Xbo of the bite retraction.
However, at this time, there is a fourth problem in which a delay may occur in the cutting system. In a general NC device, a delay of some tens of msec. occurs during a transit from the rough processing to the bite retraction or the finishing process. Although variation may be small, it is a composite delay in which a delay in the mechanical system and a delay in the electric system are combined. Also, change in grinding time constants is an addition and an unstable phenomenon of the grinding cycle such as, for example, variation in length of time required to accomplish the finishing may occur. In the case where the cycle is unstable, it is reflected by variation in processing accuracy and, therefore, adjustment is needed to reduce the cutting speed.
Hereinafter, the prior art associated with the grinding process time will be discussed.
In the practice of the grinding process, the rough grinding and the finishing grinding are carried out within one cycle to process the single work. Also, in the case of the grinding in which the processing system is of a low rigidity such as found in the internal grinder or the like, as hereinbefore discussed, the bite retraction is carried out in a small quantity after the rough processing to open the deflection to thereby decrease the time required to accomplish the finishing process.
The efficiency of the rough processing is related to the magnitude of wear or separation of the grindstone and is limited to a range in which deterioration of the processing accuracy is minimal. Although in order to reduce the processing time, the finishing allowance is to be reduced to thereby decrease the time required to accomplish the finishing, the time required to accomplish the finishing process may vary depending on a change in finishing allowance if the finishing cutting speed is slowed to secure the processing accuracy.
FIG. 13 illustrates a chart showing a processing process which is illustrated in connection with one of preferred embodiments in this specification as a novel method for setting the amount of the bite retraction after the rough processing as will be described later. With reference to this figure, problems associated with the finishing process will be discussed.
Referring to FIG. 13, when the cutting X1(t) is initiated, the processing of the work is initiated and the work dimension g(t) varies progressively. At this time, the deflection .delta.(t) is equal to X1(t)-g(t), which increases slowly and finally converges to a predetermined value. The grinding force and the grinding power P(t) are proportional to .delta.(t).
In this way, when the in-process gauge detects the work dimension having attained the finishing allowance g1, the control device commands the NC device to start the bite retraction. However, before the cutting speed changes, a delay corresponding to the time t1 during which the rough grinding takes place and the time t2 during which it stops until the bite retraction is initiated occurs. Even a delay corresponding to the time t3 required for the finishing cutting to start occurs. Nevertheless, there is a delay of the time t5 even after the termination of the grinding and before the in-process gauge detects the completed dimension g0, and therefore, the finished dimension is different from the completed dimension. Those delays are fixed for a given machine and are generally a known value.
Assuming that the grinding allowance at the time t1 is expressed by r1, the allowance at the time t2 is expressed by r2 and the allowance at the time t3 is expressed by r3, the allowance Xf(=g3) remaining after the bite retraction is expressed by the following equation: EQU Xf=g1-r1-r2-r3
The following fifth problem is found in the above discussed case.
At the time discussed above, although in the order of .mu.m, variation in amount of the bite retraction and errors in measurement by the in-process gauge are found. Even though the error is about 5 .mu.m, variation of the processing time the order of 1 sec. may result in if the finishing cutting speed is 5 .mu.m/sec. This brings about a difficulty in management of the processing site and also in standardization of the processing conditions. If the delay in cutting is large and the finishing allowance g1 is reduced, it may occur that the finishing process cannot be executed.
In the practice of the grinding job hitherto done, those inconveniences have been counteracted by increasing the finishing allowance and, on the other hand, setting the finishing cutting speed to a higher value. Also, since a delay in cutting may occur at the time of termination of the finishing process, the processing accuracy may be deteriorated if the processing resistance is high and/or if the work processing speed is high. Hitherto, a so-called spark-out grinding has been performed in which the cutting is stopped to maintain the processing accuracy. This tends to being about an unnecessary increase of the processing time.